1. Field of the Invention
The present invention relates to magnetoresistive effect element for the purpose of reading a magnetic field strength in a magnetic medium or the like as a signal, and to a manufacturing method therefor. More specifically, it relates to a magnetoresistive effect film having a large resistance change ratio in a small externally applied magnetic field.
2. Description of Related Art
In the past, there have been disclosures of magnetic reading transducers known as magnetoresistive (MR) sensors or MR heads, and it has been known that these can be used to read data from a magnetic surface with a large linear density. Such an MR sensor performs detection of a magnetic field signal via a change of resistance, which is a function of the magnetic flux-strength and direction as sensed by the reading element.
In an MR sensor such as this in the past, one component of the resistance of the reading element changed in proportion to the square of the cosine of the angle formed between the direction of the magnetization and the detection current flowing in the element, according to an operating principle known as the anisotropic magnetoresistance (AMR) effect.
The AMR effect is discussed in detail in D. A. Thompson et al "Memory, Storage, and Related Applications" IEEE Transactions of Magnetics. MAG-11, p. 1039 (1975).
Additionally, there has recently been a report of a more prominent magnetoresistive effect, whereby the resistance change in a laminated magnetic sensor is attributed to spin dependency transmission between magnetic layers with an intervening non-magnetic layer, and to accompanying spin dependency dispersion at the layer boundary.
This magnetoresistive effect is known by a variety of names, including "giant magnetoresistive effect" and "spin valve effect." A magnetoresistive sensor such as this is made from an appropriate material, and provides an improvement in sensitivity in magnetic field and an increase in resistance change when compared to observation by a sensor which uses the AMR effect.
With this type of MR sensor, the internal planar resistance between a pair of antiferromagnetic layers that are separated by a non-magnetic layer varies in proportion to the cosine of the angle formed between the magnetization directions in the two layers.
In the Japanese Unexamined Patent Publication (KOKAI) No. 2-61572, there is a description of a laminated magnetic structure which brings about a large MR effect, that occurs by means of anti-parallel orientation of the magnetization between the magnetic layers.
In the above-noted Japanese unexamined patent application publication, transition metals and alloys thereof are cited as materials that can be used in the magnetic layers in this laminated structure. There is a disclosure that FeMn is suitable for use as at least one of two magnetic layers that are separated by a center layer.
In the Japanese Unexamined Patent Publication (KOKAI) No. 4-358310, there is a disclosure of an MR sensor having a two layers of thin film ferromagnetic material which are separated by a non-magnetic metallic thin film, in which when the applied magnetic field is zero the magnetization directions in the two ferromagnetic thin films are mutually perpendicular, the resistance between the two non-coupled ferromagnetic layers varying in proportion to the cosine of the angle formed between the magnetization directions in the two layers, this being independent of direction of current flow in the sensor.
In the Japanese Unexamined Patent Publication (KOKAI) No. 6-214837, there is disclosure of magnetoresistive effect element in which, onto a substrate a plurality of magnetic thin films are laminated via an intervening non-magnetic layer, so that one of soft magnetic thin films neighbouring to each other via an intervening non-magnetic thin film, is adjacent to an antiferromagnetic thin film, wherein in the magnetoresistive effect film in which the bias magnetic field on this antiferromagnetic thin film is Hr and the coercivity of the soft magnetic thin film is Hc.sub.2 &lt;Hr, the above-noted antiferromagnetic thin film is a super lattice selected as at least two types from the group consisting of NiO, NixCo1-x, and CoO.
Additionally, in the Japanese Unexamined Patent Publication (KOKAI) No. 7-136670, in a magnetoresistive effect film having the same structure as in the Japanese Unexamined Patent Publication (KOKAI) No. 6-214837, there is a disclosure of a magnetoresistive effect element that is a two-layer film wherein onto a antiferromagnetic thin film of NiO, is laminated a layer of CoO to a thickness of 10 to 40 .ANG..
However, in a magnetoresistive effect element such as described above, although operation is by means of a small external magnetic field, a practically usable sensor or magnetic head must have a signal magnetic field applied in the direction of its easy magnetization axis, this leading to the problems that, for use as a sensor, there is no change in resistance exhibited in the area of a zero magnetic field, and that there is non-linearity occurring due to effects such as the Barkhausen jump.
Additionally, there is ferromagnetic interaction between a magnetic layers which neighbor one another via an intervening non-magnetic layer, causing the problem of a shift of the linear region of the MR curve away from the zero magnetic field.
Additionally, when using a material such as PtMn, PdMn, RhMn, NiMn, or .alpha.-Fe.sub.2 O.sub.3, which has a high Neel temperature as the antiferromagnetic thin film, thermal treatment must be performed at a high temperature, this resulting in a lowering of the resistance change ratio in the magnetoresistive effect element, which is the output when used as an element.
On the other hand, when using a material such as Ni--O, Co--O, FeMn, or IrMn, which has a relatively low Neel temperature as an antiferromagnetic thin film, the thermal stability of the device with respect to operating temperature is poor, this leading to the problem of difficulty in achieving stable element operation.
Because the structure basically obtains a change in resistance by using the change in the mean free path of conducting electrons in a three-layer structure of a magnetic thin film,/a non-magnetic thin film,/and another magnetic thin film, compared with a magnetoresistive effect film known as a coupling type, which has a multiple layer structure, there is the additional problem that the resistance change ratio is small.
In view of the above-described shortcoming of the prior art, an object of the present invention is to provide a magnetoresistive effect element film which exhibits a large linear resistance change in the region of zero magnetic field and with a small external magnetic field, and which has superior immunity to heat. Yet another object of the present invention is the improvement of manufacturability by the control of the thermal treating of the exchange coupling film.